THE CEREBRO-S FINAL AXIS 235 



are much less easily managed and regulated than is elec- 

 tricity. This agent may be applied time after time to a nerve 

 trunk without causing any permanent change in its conduc- 

 tivity, and the strength, time and duration of application, 

 etc., can be accurately governed. 



It has been noticed that the uninterrupted flow of an elec- 

 tric current through a nerve is unattended by muscular con- 

 traction; it has likewise been seen that very slow changes 

 in the strength of the current are similarly unaccompanied 

 by the manifestations of ordinary stimulation; but sudden 

 changes in the strength, whether in the direction of increase 

 or decrease, act as stimuli. However, while the passage of 

 a constant current through a nerve does not manifest itself 

 by contractions except at making and breaking, such a- pas- 

 sage brings about a change in the tissue of the nerve known 

 as electrotonus. It may be considered a state of electric 

 tension. In the anodic area the excitability is diminished 

 (anelectrotonus) ; in the kathodic area- it is increased (katel- 

 ectrotonus). Nor is the electrotonic condition restricted to 

 that portion of the nerve between the poles. Between the 

 poles there is a point where the two influences anelectro- 

 tonus and katelectrotonus meet and there is neither in- 

 creased nor decreased excitability. With weak currents this 

 point is nearer the anode ; with strong ones nearer the ka- 

 thode. A descending current diminishes the excitability of 

 a nerve; an ascending increases it. Prolonged application 

 of electric stimuli will exhaust nervous excitability, but it 

 may be restored by rest, or more quickly by an opposite cur- 

 rent. 



THE CEREBRO-SPINAL AXIS. 



The cerebro-spinal axis embraces the nervous matter in 

 the cranial cavity and in the spinal canal, excepting the roots 

 of the cranial and spinal nerves. This axis consists of both 

 white and gray matter. The white matter is made up of 

 conducting elements ; the gray matter consists of a number 



